In such clamping devices (known for example from German Pat. Specification No. 1,289,799), there is provided as driving element as a rule a driving spindle operated by means of a crank handle, the forward end of the driving spindle acting on the primary member of the power amplifier. This power amplifier may be a hydraulic power amplifier equipped with a differential piston system.
The latter comprises a primary piston having a small diameter and a secondary piston having a large diameter, the primary piston plunging into the cylinder chamber of the coaxially arranged secondary piston. Mechanical power amplifiers in this domain are also known, operating mostly on the toggle lever principle, the primary member acting on the toggle joints of a number of toggle lever pairs supported by their free end on the secondary member.
Although such clamping devices have been successful in practice, they are still in need of improvement. The driving spindle of these known clamping devices has in fact still to be turned by hand which takes time and is fatiguing for the operator.
To simplify the operation, it is also already known in clamping devices of another type to prefer, as driving element, a compressed-air cylinder, which itself is arranged in a cylinder adjoining the power amplifier housing. Since, on the one hand, the working pressure in the compressed-air mains in the finishing shops is comparatively low, while on the other hand the working pressure for actuating the primary member is comparatively high, the compressed-air piston has to have a comparatively large diameter. Owing to this large diameter, the known compressed-air clamping device cannot be used in many cases for reasons of space, in particular its use in machine vices is limited.
It is the object of the invention to provide a compressed-air operated clamping device of the aforesaid kind, which even with low working pressure has a high clamping pressure and a small external diameter, as well as being of simple construction.
This is achieved according to the invention by providing in the compressed-air cylinder, between the compressed-air piston and the primary member, a second mechanical or hydraulic power amplifier.
This second hydraulic or mechanical power amplifier enables the compressed-air piston to be kept comparatively small, even for a low operating pressure, so that the compressed-air cylinder in its external diameter is not larger or is only insignificantly larger than the cylindrical housing of the first power amplifier. This small external diameter enables the clamping device to be fitted in machine vices in which it is important to make the latter of as flat a construction as possible. Owing to the second power amplifier, the new clamping device can be connected to the usual compressed-air mains in workshops. Since for its operation, it is merely necessary to open or close valves, the clamping device can be operated effortless, and is therefore especially suitable in mass production. Despite its small external diameter, a considerable clamping pressure can be obtained with the new clamping device.
In further development of the invention, the mechanical power amplifier has at least one lever, preferably however two levers arranged symmetrically of the piston axis, each having two lever arms of different lengths, the longer lever arm bearing against the piston and the shorter lever arm acting on the primary member, the levers being mounted pivotally in an abutment connected to the cylindrical housing or to the compressed-air cylinder. This mechanical power amplifier is extremely simple in construction and permits a power amplification of the force produced by the compressed-air piston in the ratio of about 1:4.
The external diameter of the compressed-air cylinder is then only insignificantly larger than the cylindrical housing of the first power amplifier. All the levers and their bearings are arranged in the front part of the compressed-air cylinder, where they are protected from dust and spray water.
The second hydraulic power amplifier, as is known per se, may also consist of a primary piston of small diameter and a secondary piston of large diameter, the primary piston plunging into the cylinder chamber of the coaxially arranged secondary piston. This hydraulic power amplifier is still simpler in its construction, but compared with the mechanical modification has the drawback that the hydraulic fluid has to be replenished after a lengthy operational period.
Advantageously, in the hydraulic construction, the compressed-air piston and the secondary piston have the same diameter and are mounted spaced apart in one cylinder in common, an annular partition, prevented from axial displacement, being provided in the cylinder between the two pistons, the primary piston passing through the said partition. This construction is particularly simple and is also particularly space-saving, since the external diameter of the compressed-air cylinder corresponds to that of the cylindrical housing.